1. Field of the Invention
This invention relates to the field of communications, and in particular to the encoding of meta-information, such as a digital watermark, that is related to electronic content material.
2. Description of Related Art
A watermark is traditionally used as a form of verification of the source of a document. The paper used by a particular agency for currency and other negotiable instruments typically contains a watermark that is difficult to forge; a lack of the watermark is a clear indication that the instrument was not issued by that particular agency. Some watermarks are also used to verify the content of the document, for example via the use of different watermarks for each denomination of currency. Preferably, the watermark is visually unobtrusive.
A digital watermark is used to verify the source, or originator, of electronically encoded content material, such as a text document. For ease of reference, the term document is used herein to identify the content material to which the digital watermark is applied. The digital watermark is also used to verify that the document has not been altered since the time that it was encoded by the originator. That is, the digital watermark typically comprises both a digital signature, identifying the originator, and a content verifier, although it may only contain one or the other.
A digital watermark is an example of meta-information that is associated with content material. Other examples of meta-information include, for example, a summary or abstract of the content material, an index into the content material, an identifying label or tag for identification and use by another program, such as an HTML identification tag, and so on. The digital watermark is used herein as a paradigm for meta-information that is intended to be attached to the content material in a non-intrusive fashion.
A variety of techniques are available for creating a digital watermark. A common technique for verifying both the originator and the content of a document is to have the originator encrypt the document using a private key of a public-private key pair, the document being decrypted by the public key corresponding to this public-private key pair. As the names imply, the public key is known publicly, and available to any or all recipients of the document, while only the originator knows the private key. Because only the originator knows the private key, only the originator could have encrypted a document that is decryptable by the originator""s public key. Anyone having the user""s public key can decrypt the document. If a recipient changes the document, the recipient will not be able to re-encrypt the modified document such that it can be decrypted by the originator""s public key, because only the originator has the private key to effect such a re-encryption. Thus, the originator cannot deny sending the document, nor claim that the document has been altered.
The encryption of an entire document, merely to authenticate it, is a time and resource consuming process. Alternatively, the originator may merely encrypt only a portion of the document. Such an encryption verifies that the originator sent the document, but it does not verify that the content of the unencrypted portion of the document has not been altered. Alternatively, a parameter associated with the document, such as its length, or its xe2x80x9cchecksumxe2x80x9d, may be encrypted by the originator and communicated with the document.
FIG. 1 illustrates an example block diagram of a prior art watermark generator 110. The watermark generator 110 includes a processor 130 that determines a parameter 131 that is associated with a document 150, such as its length or checksum, and an encrypter 140 that encrypts the parameter 131 into a watermark 111, based on a private key 141A that is known only to the originator of the document 150. Using this watermark generator 110, the encryption verifies that the originator sent the document 150 having this particular length or checksum parameter 131. If the received document (not shown) has a different length or checksum, it must be an altered version of the document 150 that was sent by the originator. However, the receipt of a document having the appropriate length or checksum does not necessarily verify the document content, because a recipient of the original document can create an altered document having the same length or checksum, such as one that has transposed digits. In like manner the receipt of an encrypted length or checksum does not necessarily verify the originator of the document, because this watermark may merely be a copy of a watermark from another document from the originator having the same length or checksum.
Although not foolproof, the inclusion of a length or checksum is often used for a cursory content verification, similar to the numbering of pages in a printed document, wherein a missing page is evident. In like manner, the inclusion of an encrypted length or checksum is also often used for a cursory originator verification, similar to a person""s signature, which may be forged, on a printed document.
An alternative to an inclusion of a length or checksum of a document is the inclusion of a xe2x80x9chash valuexe2x80x9d of the document as the parameter 131. Upon receipt, the recipient applies the same hashing function to the document without the included hash value parameter 131, and compares the resultant hash value to a decryption of the watermark 111 that is included with the document. A preferred hashing function is one that is not easily reversible and not easily determinable. That is, the creation of a modified document that has exactly the same hash value as the unmodified document is computationally difficult, and usually infeasible, task. Thus, a hash value parameter 131 of the document 150 provides an effective verification of content. The subsequent encryption of the hash value parameter 131 using the originator""s private key 141A provides an effective verification of the originator, because a prior document is highly unlikely to have the same hash value parameter 131.
These, and other, methods for creating a digital watermark and other types of meta-information are common in the art. Typically, the meta-information is included within a separate section of the communicated document, using for example, a MIME, a HTML, or other encoding standard. A xe2x80x9ctagxe2x80x9d that identifies the start of the section, and a corresponding tag that identifies the end of the section serve to identify each section of the document, in this example, the content material and each meta-information item. A receiving application that is compatible with the encoding standard segregates the document and processes each section accordingly. For example, the material between a xe2x80x9cBegin Textxe2x80x9d tag and an xe2x80x9cEnd Textxe2x80x9d tag in a document will be displayed on a display device; the material between a xe2x80x9cBegin Watermarkxe2x80x9d tag and an xe2x80x9cEnd Watermarkxe2x80x9d tag will be processed to verify either the content or the originator, or both, and a corresponding document, such as xe2x80x9cThis document is verified as having been communicated from Alicexe2x80x9d will also be displayed, or rendered as a vocal message.
A device that is not compatible with the particular standard, however, will not recognize the segmentation tags, and will typically display the document, with the included meta-information and the aforementioned tags, on a display device. Although the text will, in general, be recognizable, the inclusion of a display of the meta-information and tags is visually unappealing. In many cases, the display of this xe2x80x9cextraneousxe2x80x9d information has the effect of making the text of the document unrecognizable to a recipient who is not accustomed to the raw form of computer encoded documents. As noted above, an ideal metainformation item in an electronic document, on the other hand, is one that is visually unobtrusive.
It is an object of this invention to provide a means for including a substantially xe2x80x9cinvisiblexe2x80x9d digital watermark, or other meta-information, within an electronic document. It is a further object of this invention to provide a means for encoding meta-information into an xe2x80x9cinvisiblexe2x80x9d form.
These objects and others are achieved by encoding meta-information, such as a digital watermark, using characters that are not visually apparent when the document is rendered, and attaching this encoded meta-information to the document. Characters such as spaces, tabs, carriage returns, backspaces, line feeds, and so on are used to encode the meta information. A receiving device that is compatible with this encoding is able to decode and process the meta-information; for example, to decode a watermark and verify the contents and/or the originator of the document. A receiving device that is not compatible with this encoding will merely display or print additional xe2x80x9cwhite spacexe2x80x9d. Preferably, by appending this invisible encoding to the document, this additional white space will occur after the display or printing of the document, in a visually unobtrusive form.